Often in drilling an oil or gas well, drilling fluids, (commonly referred to as “mud”) are circulated through the wellbore. The drilling fluids operate to convey cuttings generated by a drill bit to the surface, drive a down-hole drilling motor, lubricate bearings and a variety of other functions. Wellbore telemetry systems are often provided to transmit information from the bottom of a wellbore to the surface of the earth through the column of drilling fluids in a wellbore. This information might include parameters related to the drilling operation such as down-hole pressures, temperatures, orientations of drilling tools, etc., and/or parameters related to the subterranean rock formations at the bottom of the wellbore such as density, porosity, etc.
Telemetry systems generally include a variety of sensors disposed within a wellbore to collect the desired data. The sensors communicate with a transmitter, such as a mud pulse tool (MP tool), also disposed within the wellbore. The MP tool might, e.g., be configured to generate known patterns of pressure fluctuations in the mud stream that correspond to the sensed data. The patterns of pressure fluctuations travel as waves to a receiver at the surface of the wellbore where pressure sensors associated with the receiver measure pressure fluctuations as a function of time. A decoder is generally provided in communication with the receiver to interpret the pressure measurements made at the surface.
The pressure waves generated by an MP tool are subject to attenuation, reflections, and noise as they move through the mud stream. For example, a first pressure wave generated by an MP tool might be reflected off the bottom of the wellbore and be superimposed with a second pressure wave generated by the MP tool as the pressure waves travel up the wellbore. Also, the pressure waves transmitted by the MP tool may combine with noise sources such as pressure waves generated by mud pumps at the surface, or by various down-hole components such as a drilling motor, and a drill bit interacting with the subterranean rock formation being drilled. These factors tend to degrade the quality of the signal and may make it difficult to recover the transmitted information.
Various methods may be employed in an attempt to reduce the interfering effects of the external factors in a telemetry system, and to more accurately interpret the data received.